This invention concerns a device for the electrostatic application of material particles entrained in a stream of gas to an advancing, flat substrate in the form of webs or sheets, particularly made of paper, in printing machines or the like.
In a known device of this type (German Patent Disclosure No. 2,646,798), the charging of powder particles takes place in an air jet head, so that powder particles therefore leave this head with an electrical charge and are deflected to the substrate to be coated. The substrate to be coated is generally an electrically nonconductive material, such as paper or textile. In this case, a counterelectrode is located beneath the substrate to be coated with powder.
Aside from a frequently inadequate electrical charging of the powder particles in the jet head, the known device has the drawback that the particles already charged in the jet head are deflected not only towards the substrate but also towards the machine frame, where the powder is deposited and forms troublesome deposits. In particular, these deposits in the dusting of paper sheets or the like can also occur on the feed system transporting the sheets, consisting of clips or the like, whereby the feed system loses its operating capability. Frequent shutdowns of the machine are necessary for the purpose of cleaning. In addition, the powder material which has been deposited on the feed system or on the machine frame, ordinarily at ground potential, is not reusable and is lost for its intended purpose.
The purpose of this invention is to propose a type of device which leads to less soiling and a saving of powder material to be applied.
This purpose is accomplished pursuant to the invention by the following features:
(a) The gas stream carries electrically neutral material particles with it;
(b) beneath the advancing substrate, in the area of the point of impingement of the gas stream on the substrate, is located at least one charging electrode for the production of a corona discharge;
(c) at least one counterelectrode is located in the advance direction of the substrate at a distance from the charging electrode.
The core of the invention therefore consists of the fact that the particles to be applied obtain their electrical charge only in the immediate vicinity of the substrate and can then only deposit on the substrate practically exclusively, but not on the machine frame, feed system, or the like. In this way, a soiling of a printing press or the like is prevented, and a material saving of up to 50% can be achieved since the material to be applied is not deposited at other places.
The following description of preferred forms of embodiment of the invention provide further explanation in connection with the attached drawing.